1. Field of the Invention
The present invention relates to a switching power supply device for supplying a stabilized low DC voltage to an industrial or household electronic apparatus.
2. Description of the Prior Art
Recently, in the field of manufacturing electronic apparatuses, greater attention has been directed toward cost reduction, compactness, higher equipment performance, and energy saving. Accordingly, strong needs exist for a switching power supply device which is more compact and has higher output stability and higher operating efficiency.
Among conventional switching power supply devices, a self-excited flyback type switching power supply device has been widely used, because it consists of fewer parts and can be manufactured at a relatively low cost. It is known, however, that the switching frequency of such a device fluctuates depending upon degree of the output current, with the result that interferences occur in the operation of an associated electronic apparatus and that a larger size rectifying and smoothing circuit is required.
In an attempt to overcome such problems with the prior art devices, a switching power supply device of the regenerative control type has been proposed. FIG. 7 shows a switching power supply device of such at type. The device of FIG. 7 comprises a DC power source 1, a transformer 3, switching elements 4 and 14, a diode 5, a synchronizing oscillation circuit 6, a rectifying diode 7, a smoothing capacitor 8, a secondary switching element 14, and a control circuit 15.
The DC power source 1 rectifies an AC voltage and smoothes the resulting DC voltage. Alternatively it may consist of a battery or the like. The positive and negative terminals of the DC power source 1 are connected to input terminals 2 and 2', respectively. The transformer 3 has: a primary winding 3a connected at one terminal to the input terminal 2 and at the other terminal to the input terminal 2' through the switching element 4; a secondary winding 3c connected at one terminal to an output terminal 10' and at the other terminal to an output terminal 10 through the rectifying diode 7; and a bias winding 3b connected at one terminal to the input terminal 2' and at the other terminal to the synchronizing oscillation circuit 6. The switching element 4 turns ON or OFF in response to ON/OFF signals which are supplied at the control terminal from the synchronizing oscillation circuit 6, thereby applying the input voltage to the primary winding 3a and interrupting the voltage. The synchronizing oscillation circuit 6 actuates the switching element 4 to turn ON and OFF respectively for predetermined ON and OFF periods of time. The OFF period continues until the polarity of the induced voltage of the bias winding 3b is inverted. This repeated ON/OFF cycle causes continuous oscillation.
The energy accumulated in the transformer 3 during an ON period of the switching element 4 is released from the secondary winding 3c through the rectifying diode 7 or the switching element 14 to the smoothing capacitor 8 during an OFF period of the switching element 4. After the above-mentioned energy release, a secondary current is caused to flow reversely from the smoothing capacitor 8 to the secondary winding 3c through the switching element 14. The period of this reverse flow is controlled by the control circuit 15. The rectifier diode 7 is connected at its anode to one terminal of the secondary winding 3c and at its cathode to the output terminal 10. The smoothing capacitor 8 is connected between the output terminals 10 and 10'. The induced voltage appearing across the secondary winding 3c is rectified by the rectifying diode 7, and then smoothed by the smoothing capacitor 8 to provide an output voltage. The control circuit 15 compares the voltage appearing across the output terminals 10 and 10' with an internal reference voltage in order to vary the above-mentioned flow period of the secondary current through the secondary switching element 14.
The operation of the power supply device is described with reference to FIG. 8. In FIG. 8, (a) shows the waveform of a voltage V.sub.DS appearing across the switching element 4; (b) the primary current I.sub.D flowing through the primary winding 3a; (c) the waveform of driving pulse V.sub.G1 output from the synchronizing oscillation circuit 6; (d) the secondary current I.sub.D flowing through the secondary winding 3c; and (e) the waveform of a driving pulse V.sub.G2 for the secondary switching element 14. The hatched portions in (e) of FIG. 8 indicate reverse flow periods for causing the secondary current I.sub.o to reversely flow in the secondary winding 3c.
A magnetic flux develops in the transformer 3 as the primary current I.sub.D flows through the primary winding 3a during an ON period of the switching element 4 which period is determined by the synchronizing oscillation circuit 6, so that energy is accumulated in the tranformer 3, whereupon an induced voltage develops in the secondary winding 3c. It is so arranged that the rectifying diode 7 is reversely biased by the induced voltage and that the switching element 14 remains in its OFF-position. When the switching element 4 turns OFF in response to an OFF signal from the synchronizing oscillation circuit 6, a fly-back voltage develops in the primary winding 3a, and, simultaneously, a fly-back voltage is induced in the secondary winding 3c in such a direction that the rectifying diode 7 is forward biased. Accordingly, the energy accumulated in the transformer 3 is released as a secondary current I.sub.o through the secondary winding 3c, which is then smoothed by the smoothing capacitor 8 and supplied as an output voltage to the output terminals 10 and 10'. In this case, the switching element 14 is actuated to turn ON by the control circuit 15, but there occurs no particular operation change, through whichever the diode 7 or the switching element 14 the secondary current may flow.
When all the energy accumulated in the transformer 3 is released until the secondary current becomes zero, the voltage appearing across the smoothing capacitor 8, that is, output voltage, is applied to the secondary winding 3c through the switching element 14 which has been already in ON state, and accordingly the secondary current flows reversely from the smoothing capacitor 8, so that a magnetic flux is generated in the reverse direction in the transformer 3, thereby causing energy to be accumulated therein. In this condition, there is no change in the polarity of the induced voltage developing in each winding of the transformer 3, therefore, there is no change in the fly-back voltage in the bias winding 3b. Accordingly, the synchronizing oscillation circuit 6 causes the switching element 4 to remain in the OFF state.
As mentioned above, the ON period of the switching element 14 is controlled by the control circuit 15. When the switching element 14 becomes OFF, the induced voltage in each winding of the transformer 3 is inverted in polarity. Therefore, the induced voltage developing in the secondary winding 3c causes the rectifying diode 7 to be reversely biased. Since the switching element 14 is in the OFF state, the secondary winding current does not flow. In the primary winding 3a, the induced voltage develops in such a direction that the voltage at the terminal to which the switching element 4 is connected is negative, and, on the other hand, the voltage at the terminal to which input terminal 2 is connected is positive. Therefore, the primary current flows in such a direction that the DC power source 1 is charged through the diode 5, so that the energy accumulated in the transformer 3 during the OFF period is supplied to the DC power source 1 (i.e., the power regeneration is conducted). At this time, the polarity of the induced voltage developing in the bias winding 3b is also inverted, and accordingly the synchronizing oscillation circuit 6 actuates the switching element 4 to turn ON. In this case, there is no particular operational change, through whichever the diode 5 or the switching element 4 the primary current may flow.
When all the energy accumulated in the transformer 3 during the OFF period is released until the primary current is reduced to zero, the primary current flows from the DC power source 1 through the switching element 4 which has been already in the ON state, so that the transformer 3 is charged in the direction contrary to the that of the above-mentioned discharge, with the result that a magnetic flux develops in the transformer 3 and energy is thus accumulated therein. In this condition, there is no change in the polarity of the induced voltages developing in each windings of the transformer 3, and accordingly the synchronizing oscillation circuit 6 maintains to control the switching element 4 to be kept in the ON state.
When the switching element 4, the ON period of which is determined by the synchronizing oscillation circuit 6, is actuated to turn OFF, the energy accumulated in the transformer 3 is released as the secondary current through the secondary winding 3c. Cycles of these operations are repeated so that the output voltage is continuously supplied across the output terminals 10 and 10'.
The manner of performing the steady control of the output voltage will be described. In FIG. 8 showing the waveforms at various portions of the power supply device of FIG. 7, the OFF period (between times t.sub.1 and t.sub.3) of the driving pulse V.sub.G1 in the synchronizing oscillation circuit 6 is represented by T.sub.OFF, the reverse flow period (between times t.sub.2 and t.sub.3) of the secondary current I.sub.o is represented by T'.sub.OFF, the ON period (between times t.sub.3 and t.sub.5) is represented by T.sub.ON, and the regenerative period (between times t.sub.3 and t.sub.4) of the primary current I.sub.D is represented by T'.sub.ON. Then, the current I.sub.OUT output from the output terminals 10 and 10' can be expressed by: ##EQU1## The output voltage V.sub.OUT can be expressed by: ##EQU2## The oscillation frequency f is expressed by; ##EQU3## In the above expressions, N.sub.S represents the number of turns of the secondary winding 3c; N.sub.p represents the number of turns of the primary winding 3a; L.sub.S represents the inductance of the secondary winding 3c; and V.sub.IN represents the input voltage supplied from the DC power source 1.
The ON period T.sub.ON is kept at a constant value determined by the synchronizing oscillation circuit 6. If the output voltage V.sub.OUT is constant, therefore, the OFF period T.sub.OFF is constant, and the oscillation frequency f is also constant. However, the reverse flow period T'.sub.OFF may be varied by the secondary switching element 14 controlled by the control circuit 15, and K=(1/2).multidot.V.sub.OUT .multidot.(1/L.sub.S).multidot.(T.sub.OFF /T) in the expression (1) is constant when the output voltage V.sub.t is constant. Even if the output current I.sub.OUT is varied, therefore, it can be controlled by changing the reverse flow period T'.sub.OFF. Even if the input voltage V.sub.IN is varied, moreover, it can be controlled by changing the reverse flow period T'.sub.OFF, as seen from the expression (2). Therefore, the output voltage V.sub.OUT can be controlled so as to be always kept constant by changing the reverse flow period T'.sub.OFF. The reverse flow period T'.sub.OFF can be changed by controlling the ON period of the switching element 14 which is controlled by the control circuit 15.
FIG. 9 shows various waveforms obtained when the output current I.sub.OUT is changed. In FIG. 9, solid lines indicate waveforms obtained when the output current I.sub.OUT flows at the maximum level from the output terminals 10 and 10', or at the so-called maximum load period, and broken lines indicate waveforms obtained when the output current I.sub.OUT is zero, or at the so-called no-load period. When the input voltage is constant, the ON period T.sub.ON is constant, and accordingly flux variation range .DELTA.B is always constant.
In such a regenerative control type switching power supply device, when the switching element 4 turns OFF, a surge voltage occurs due to the leakage inductance in the transformer 3. At the maximum load, the level of the surge voltage is approximately the same extent as in a conventional self-excited fly-back type switching power supply device, and, at a light load, the level of the surge voltage is greater than that caused in such a self-excited fly-back type power supply device because the peak value of primary current is considerably higher immediately before the turn OFF. The conventional regenerative control type switching power supply device provides an advantage that because of its ability to regenerate energy at the turn ON of the switching element 4, even when a snubbing capacitor is connected between the both terminals of the switching element 4, the surge voltage at the turn-on can be efficiently restrained without involving any turn-on loss. However, the resonance energy due to the capacitor and the leakage inductance of the transformer 3 is considerably great, so that ringing waveforms are superposed over another throughout each OFF period, which becomes a source of noise. The addition of a larger snubbing capacitor becomes a greater hindrance to realization of a higher switching frequency for compactness of the power source.
When the input DC power source is obtained from an AC power source through a rectifying and smoothing circuit, it is usual that the rectifying and smoothing circuit is of the capacitor input type which comprise a smoothing element and a capacitor and that the capacitor is used as an input capacitor which serves as a DC power source. Generally, it is desired that power supply devices have a higher efficiency and be small in size, and on the other hand the output holding time is required to be set in order to protect electronic apparatuses as a load from possible troubles such as momentary interruption of input power. In the prior art arrangement, the output holding time depends largely upon the static capacitance of the input capacitor, and, therefore, the static capacitance is determined by both the power capacity of the power source and the output holding time. For this reason, even when there is a sufficient ripple withstand capacity, there are cases where use of a larger input capacitor is required. In addition, the conducting period of the input current from the AC power source is shorter in the stage of steady operation so that the peak value of the input current becomes larger, thereby causing the problem in that the power factor and efficiency substantially drop.